An image sensor is a device which senses light of an optical image and then converts it into the form of an electrical signal. Image sensors have developed from video camera tubes and nowadays to CCD or CMOS devices, which are used in modern cameras for consumer environments including DSLR and compact cameras, cameras for mobile telephones, webcams and optical mice; and industrial applications such as machine vision and automotive fields.
CCD's have historically been the preferred choice for image sensors in high performance digital cameras because they have historically been able achieve higher noise performance, with a lower dark current than their CMOS counterparts. However, recent advancements in CMOS noise performance have enabled CMOS sensors to become prominent in high performance cameras.
CCD's begin by projecting light onto a capacitor array which charges the individual capacitors. The charge on the first row is then transferred to a read out register and then onto an amplifier and analog to digital convertor (ADC). Once the first row has been read, its charges are deleted and the above row's charge is transferred down. This can be seen as a domino effect so all the above charges are moved down one row to fill the space. From then on the same process will read out the charge row by row to process the whole image.
CMOS image sensors are made up of an array of photo detectors which are contained inside the structure of a pixel. Light energy is taken from the photodiode and converted to charge by the CMOS pixel. The charge is then transferred and converted into a voltage with the use of transistors acting as switches. The signal can then be filtered by switched capacitor circuits and then processed by an ADC.
CMOS imagers also excel in the speed category. All camera functions can be integrated into the image sensor so signal distances are shorter with less capacitance, inductance and propagation delays.
For these and other reasons, CMOS sensors are of increasing commercial importance. In this fast moving and growing technical and commercial field it is therefore desirable to further improve the noise performance of CMOS image sensors.